Studies on the antiviral proteins from many different plant species have been carried out, starting from the discovery of pokeweed antiviral protein (or Phytolacca antiviral protein, hereinafter referred to as "PAP") isolated from a crude extract of Phytolacca americana L. (see: Irvin, J. D., Arch. Biochemistry Biophys., 169:522-528 (1975)). In addition to PAP, a few antiviral proteins have been isolated from several plants, e.g., Ricin (from Ricinus communis) (see: Halting, K. C. et al., Nucleic Acid Res., 13:8019-8033 (1985)), Mirabilia antiviral protein ("MAP", from Mirabilia jalapa L.) (see: Kataoka, J. et al., J. Biol. Chem., 266:8426-8430 (1991)) and .alpha.-trichosanthin (from Trichosanthes kirilowii) (see: Zhang, X. et al., Nature, 321:477-478 (1986)). Said antiviral proteins have been reported to be ribosome inactivating proteins ("RIPs") having RNA N-glyoosidase activities (see: Endo, Y. et al., J. Biol. Chem., 263:8735-8739 (1988)).
In general, PAP from Phytolacca americana U is classified as PAP-I, PAP-II and PAP-S that appear in spring leaves, summer leaves, and seeds, respectively; and it is reported that antiserum reactions of these PAPs are different from one another (see: Irvin, J. D. et al., Arch. Biochemistry Biophys., 200:418-425 (1980)). Further, it has been known that ribosomes of Phytolacca americana L. are depurinated by the RNA N-glycosidase activity of PAP. On the other hand, an immunoconjugate of PAP with CD4 or CD19 has been reported to inhibit the replication of human immunodeficiency virus type I (see: Jansen, B. et al., Cancer Res., 52:406-412 (1992); Kim Y. W. et al., J. Immunol., 144:1257-1262 (1990)); Myers, D. E. et al., J. Immunol. Methods, 136:221-238 (1991)); in this connection, said PAPs have been proposed to be applicable to the treatment of AIDS. Accordingly, molecular biological studies on the PAPs have been actively carried out, including the nucleotide sequence analysis of cDNA of PAP; the elucidation of the precise mechanism of PAP's biological activity; construction of a transgenic plant; and, application to the immunoconjugate preparation.
The present inventors first developed a transgenic plant expressing PAP, and patent applications covering said expression vector are pending under the title "Expression vector for Phytolacca antiviral protein and process for preparing transgenic plant transformed thereof" (U.S. Ser. No. 08/049,075; EP appln. No. 93 110 445.9; JP appln. No. 5-128222). However, it is clear that the expression vector of the prior art is simply designed for the purpose of transformation of plants to confer viral resistance, in light of employing the CaMV 35S promoter which is generally used for the expression of plant genes.
Accordingly, the expression of PAP gene has been restricted to plants; and, therefore, there is a need in the art for the development of a practical expression vector which produces PAP at high levels in a versatile microorganism.